The present invention relates to self-adjusting blades for engagement with a moving work surface, said blades being useful for coating, creping, doctoring and other scraping operations in the printing industry, in flexogravure or rotogravure techniques.
Although the present invention is not restricted hereto it will in the following be described mainly in relation to the coating of paper substrates.
Blades used in conventional coating techniques are usually made of different types of materials. Among such materials there may be mentioned high-strength, hardened and tempered carbon steels, blade substrates covered at the edge or tip with ceramic hard wear-resistant materials, such as described in British patent 2 130 924, and low alloyed steel with local hardening of the edge section, as described in EP 0 672 761.
Blades made of hardened and tempered carbon steel exhibit quite poor wear resistance behaviour and have to be replaced frequently in view of the abrasive wear caused by the base paper and the coating colour pigments. Their hardness is typically within the range 500 to 600 HV depending on the thickness of the steel strip.
On the other hand the low abrasion resistance of such steel blades allows a short self-adjusting period when installed in a coater machine. This makes the blade easy to use and non-sensitive to the exact coater setting or to existing unevenness in geometrical conditions along the blade holder. This is especially important for coating using stiff blade mode, i.e. when the angle between the tip of the blade and the paper on the coater is high, usually 10xc2x0 or more.
Another feature of carbon steel blades is their behaviour of wear at the site of coating colour entrance in stiff blade mode. According to the literature (Schachtel et al., Wochenblatt far Papierfabrikation 16-1993, p 661-667) a round wear form can be obtained (see FIG. 1 of the literature reference). A small but visible radius (r) is formed at the entrance site of contact between the blade and the base paper. This radius results by the combination of erosive effect of the coating colour impingement and the abrasive effect of the paper fibres. Such feature is of primary interest for rotogravure type of coating recipe, where the pigments are mainly constituted by platelets with a high shape factor. The existence of such a radius (r) assists in the proper orientation of the coating colour pigments before passing beneath the blade resulting in optimum printability characteristics.
Hard material tipped blades, such as blades with a ceramic coating, as well as edge section hardened low alloy steel blades perform better than carbon steel blades in terms of life period. Blades tipped with hard material exhibit typical hardness values of the tip in the range from 900 to 1200 HV, while the locally hardened edge section of low alloy steel blades reaches about 800 HV, the rest of the blade reaching about 600 HV.
Although the wear resistance property is an important factor in the industrial interest for such blades, such property is at the same time a limitation in their use in view of the necessity to adapt specifically each tip design according to the exact running condition of the blade and the setting of the blade holder in the coating machine. The high wear resistance does not allow incorrect setting because it will take too long to adjust the bevel in a running-in period. This is normally not acceptable in industrial coating conditions and could result in poor MD and CD profiles of the coated paper and/or poor surface quality. Furthermore, the rounding of the entrance point as described above will not be formed as readily.
The features described above form the basis for resolving the problems encountered with the prior art and the invention seeks to provide a solution wherein the advantages of using materials of high wear resistance are combined with the advantages of using materials of lower wear resistance.
One object of the invention is, accordingly, to provide a blade which will behave similarly to a carbon steel blade when loaded and during the running-in period, i.e. obtaining self-adjusting performance of the blade.
Another object of the invention is to provide a blade which after a short running-in period will behave in the same way as a locally hardened edge section of a low alloy steel blade resulting in high wear resistance performance.
Still another object of the invention is to provide a blade capable of wear to result in a rounded entrance contact site, with the major part of the metering surface in contact with the base paper and the coating colour performs similarly to low alloy steel blades with a local hardened section.
For these and other objects which will be clear from the following disclosure the invention provides for a self-adjusting blade for engagement with a moving work surface. The blade comprises a steel strip elongated in a first direction between first and second sides, said strip having an edge section along said first side for contact with said work surface, and said edge section being hardened to a hardness exceeding that of the remaining part of said strip. Said second side is intended for attachment to a blade holder in a conventional manner. The blade according to the invention is characterized by a coating of a low wear resistance material covering substantially all of said edge section at least on the part thereof contacting the work surface.
According to one embodiment of the invention said steel strip is constituted by a low alloyed steel hardened to a hardness of between about 400 and 600 HV, said edge section being further hardened to a hardness of between about 700 and 900 HV.
A preferred embodiment of such blade is one wherein said steel strip is constituted by a cold rolled hardened and tempered strip having the composition (percent by weight):
C 0.46-0.70;
Si 0.2-1.5;
Mn 0.1-2.0;
Cr 1.0-6.0;
Mo 0.5-5;
V 0.5-1.5;
B greater than 0.01;
Nixe2x89xa61.0;
Nbxe2x89xa60.2.
The material of low wear resistance has suitably a hardness between about 200 and 600 HV. Suitable materials are pure metals, alloys, oxides, polymers, or mixtures of two or more thereof.
It is particularly preferred that said material of low wear resistance is selected from molybdenum containing up to 4% O2, Ni- or Co-based alloys, Cu-based alloy, AlSi/polyester blends or Co-base polymer blends, or stainless steel.
For ease of adaptation to the moving surface the edge section of the blade is preferably provided with a bevel on the side thereof contacting the moving surface.
The thickness of the blade substrate can vary from about 0.15 to about 0.8 mm. The thickness of the self-adjusting coating suitably lies within the range about 1 to about 100 xcexcm, preferably 20 to 50 xcexcm.